The first change which milk undergoes in the stomach is coagulation. The casein and butterfat separates from the serum, which is soon absorbed unchanged into the blood, according to the laws of diffusion. [Quoting Wagner's Manual of Physiology.]

1875, A[lphonse René le Mire de] Normandy; Henry M[inchin] Noad, “Butter”, in The Commercial Hand-book of Chemical Analysis; or Practical Instructions for the Determination of the Intrinsic or Commercial Value of Substances Used in Manufactures, in Trades, and in the Arts, new edition, London: Lockwood & Co., 7 Stationers'-Hall Court, Ludgate Hill, OCLC14861950, page 95:

By submitting pure solid butter-fat to powerful pressure in a linen cloth at the temperature of 60°, a slightly yellow transparent oil will flow out, and a solid white fat will remain behind; the first is called oleine or butter-oil, and the second margarine; and of these two substances the pure fat of butter almost entirely consists, although, as may be supposed, their relative proportions are liable to great variations.

During the ripening the cream should be stirred thoroughly and frequently. This insures uniformity and prevents the butter fat separating from the milk constituents of the cream. It was mentioned previously that bacteria do not act upon the butter fat, but upon the other constituents. If the butter fat rises to the top of the cream, as it will if not stirred, the flavor and aroma will not be uniformly absorbed.

1947, The Indian Journal of Veterinary Science and Animal Husbandry, Calcutta: Central Publication Branch, Government of India, OCLC6267236, page 182:

From the analytical data in Table I it will be seen that goat and sheep butterfats have almost similar characteristics to those of cow and buffalo butterfats. Goat and sheep butterfats have a high Polenske value and low iodine value.

Much has been written in both technical and popular publications about factors which affect butterfat content of milk. It is well known that the breed and individuality of the cow, stage of lactation, and many other factors affect individual cows or all cows in a herd, so that the butterfat percentage changes from milking to milking.